1. Field of the Invention
The present invention relates to an apparatus for driving an image pick-up device, and more precisely it relates to an imaging device driving apparatus which can control the accumulation time of electrical charges of an imaging device, such as a CCD image pick-up device in a still video camera or the like.
2. Description of Related Art
Recently, various electronic still cameras having CCD image pick-up devices used therein in place of conventional silver halide films have been developed. In such electronic still cameras, the shutter speed is mechanically controlled by means of a mechanical shutter that is similar to that found in cameras in which the silver halide film is used to control the period of time in which a light receiving portion of the CCD pick-up device receives light of an object to be photographed.
In a CCD pick-up device, a shutter speed control function similar to the conventional mechanical shutter can be realized by controlling the interval of time for transferring electrical charges accumulated in a light receiving portion (photodiode) to a vertical transfer portion (CCD), i.e., the accumulation time. This is the reason that a pure electronic shutter, which controls the time for accumulation of electrical charges, has been widely adopted in a camera to decrease the weight thereof.
However, in a CCD image, pick-up device which is used in a conventional electronic still camera, transfer pulses are periodically outputted to transfer the signal charges accumulated in the light receiving portion to the vertical transfer portion. Therefore, an apparatus for outputting the transfer pulses at an optional time must be additionally provided. However, the provision of such an additional device makes the electronic control circuit complex and increases the manufacturing cost thereof.
Furthermore, since no mechanical shutter is provided in a pure electronical shutter, light of an object is always incident upon the CCD image pick-up device. This causes a so-called brooming smear phenomenon especially at high object brightness.
The brooming is a phenomenon is caused when an intensive light is incident upon an image pick-up element. The electrical charges overflow into circumferential light receiving elements or the vertical transfer portions, so that a bright portion circumferentially spreads. Smear is a phenomenon caused when signal charges are transferred to the vertical transfer portion. Unnecessary electrical charges overflow from the light receiving element, on which the intensive light is made incident, into the vertical transfer portion, so that vertically extending bright fringes occur.
In particular, in an inter-line transfer type of CCD image pick-up device, which is mostly used, it takes a relatively long time to read the signal charges transferred to the vertical transfer portion upon completion of the shutter operation. Accordingly, unnecessary electrical charges accumulated in the light receiving portion tend to overflow into the vertical transfer portion, and are added to the signal charges, resulting in the occurrence of the smear phenomenon.
Generally speaking, brooming occurs more or less in conventional silver halide film, and the to some extent can be accepted to form a natural image. On the other hand, smear phenomenon in the vertical direction leads to an unnatural image, resulting in the failure of a picture.
Furthermore, since the CCD image pick-up device is always exposed, as mentioned above, unnecessary electrical charges accumulated in the light receiving portion must be periodically discharged when no picture is taken. To this end, electrical charges of the light receiving portion are transferred to the vertical transfer portion at one time in accordance with transfer pulses which are periodically outputted by an image pick-up element driver. Thus the electrical charges transferred to the vertical transfer portion are swept to discharge the same into a drain or a silicon circuit board.
Upon operating the shutter, the transfer pulse is first outputted from the image pick-up element driver at a time which is obtained by a calculation of the shutter speed (exposure time, accumulating time), after the periodical generation of the transfer pulses, to transfer the unnecessary electrical charges of the light receiving portion to the vertical transfer portion. As a result, no electrical charge exists in the light receiving portion, so that the accumulation of the signal charges can be commenced. The unnecessary electrical charges of the vertical transfer portion are swept or discharged in accordance with a sweep signal before the subsequent periodical transfer pulse is outputted, i.e., within an exposure time.
After the expiration of the exposure time, the periodical transfer pulse is outputted to transfer the signal charges accumulated in the light receiving portion to the vertical transfer portion, so that the signal charges can be read in accordance with a read signal and be recorded.
As can be understood from the above discussion, in a conventional electronical shutter, the time for sweeping or discharging the unnecessary electrical charges decreases as the shutter speed increases, i.e., as the exposure time decreases. Accordingly, the sweep or discharge of the unnecessary electrical charges must be effected at a high speed.
However, since the capacity of the gate electrodes of the vertical transfer portion is large, if the frequency of the sweep signal is increased, the transfer efficiency is reduced. Namely, the amount of electrical charge which can be swept at one time is decreased.
On the contrary, in the case of a high speed shutter, since an object to be photographed is usually bright, there is a large amount of light that is incident upon the light receiving portion. In addition, the time for accumulating the unnecessary electrical charges increases as the shutter speed increases, and accordingly, the quantity of unnecessary electrical charges to be accumulated increases. Therefore, there is a difficulty in completely sweeping the unnecessary electrical charges when the shutter speed is high. As a result, the unnecessary electrical charges which are not swept are added as a noise to the signal charges at high shutter speed, resulting in damage to a picture.
For instance, in a CCD image pick-up device in which the periodical transfer pulses are outputted at an interval of 1/60 second, when the shutter speed is 1/125 second, the unnecessary electrical charges are accumulated in the light receiving portion for only about 1/125 (1/60-1/125) second. Accordingly, the unnecessary electrical charges transferred to the vertical transfer portion can be fully swept, resulting in the formation of a noiseless picture having a uniform brightness.
However, in the case of a high shutter speed, such as 1/2000 second, unnecessary electrical charges are accumulated in the light receiving portion for about 1/60 (1/60-1/2000) second. In addition, generally speaking, since the brightness of the object is high at the high shutter speed, the amount of unnecessary electrical charges accumulated in the light receiving portion increases by several times that of the 1/125 second shutter speed mentioned above. Therefore, the unnecessary electrical charges can not be completely swept, so that the unnecessary electrical charges which can not be swept remain in the vertical transfer portion. Consequently, the residual unnecessary electrical charges are added to the signal charges, so that a lower portion of the picture becomes brighter than the remaining portion thereof.
Furthermore, in a conventional electronic still camera, a CCD image pick-up unit (an inter-line transfer type of CCD image pick-up element and drive thereof) equivalent to that used in a video camera is used to reduce the manufacturing cost.
In a CCD image pick-up unit for a video camera, the signal charges which are accumulated in the light receiving portion are periodically (about every 1/60 second) transferred to the vertical transfer portion at one time at a normal photographing mode (movie mode), so that the signal charges are successively read prior to a periodical transfer of subsequent signal charges. The transfer of the signal charges to the vertical transfer portion is effected in accordance with periodical transfer pulses which are outputted by a pulse signal output device, and the reading of the signal charges is effected in accordance with read pulses having vertical and horizontal transfer pulses which are outputted by the pulse signal output device. Namely, in the movie mode, all the operations are controlled by the pulses periodically outputted by the pulse signal output device.
On the other hand, in the operation of the electronical shutter (a still mode), unnecessary electrical charges which are accumulated in the light receiving portion are first transferred to the vertical transfer portion in accordance with a first compulsive transfer pulse which is outputted by a control device before the first periodical transfer pulse immediately after the output of the read signal is outputted, and the unnecessary electrical charges are swept in accordance with a sweep pulse signal which is outputted by the pulse signal output device.
After that, the read pulse which is outputted by the pulse signal output device is stopped, so that the unnecessary electrical charges accumulated in the light receiving portion are transferred to the vertical transfer portion in accordance with a second compulsive transfer pulse which is outputted by the control device at a predetermined time to commence the exposure. The unnecessary electrical charges which are transferred to the vertical transfer portion by the second compulsive transfer pulse are swept within a short space of time in accordance with the sweep pulse signal which is outputted by the pulse signal output device before the expiration of the exposure time.
Upon completion of exposure, the periodical transfer pulses are outputted from the pulse signal output device, so that the electrical charges accumulated in the light receiving portion during exposure are transferred to the vertical transfer portion. After that, the signal charges are read by the periodical read signals which are outputted by the pulse signal output device.
The operation of the conventional image pick-up device mentioned above will be explained below with reference to FIGS. 21 and 30. FIGS. 21 and 30 are timing charts of a still mode and a movie mode, respectively. In the illustrated examples, periodical transfer pulses CTG and read signals are periodically outputted at a predetermined interval (about 1/60 second). The output duration of the periodical transfer pulses CTG will be referred to as a field hereinafter.
In the still mode, a first compulsive transfer pulse PTG1 and a sweep signal are outputted after the read signal is outputted and before the periodical transfer pulse CTG is outputted, so that the unnecessary electrical charges accumulated in the light receiving portion are transferred to the vertical transfer portion and are swept. Namely, the sweeping of the unnecessary electrical charges is effected in field n immediately before the field n+1 in which exposure time for accumulating the signal charges for recording the picture is included.
After that, at a predetermined time, a second compulsive transfer pulse PTG2 is outputted, so that the unnecessary electrical charges accumulated in the light receiving portion are transferred to the vertical transfer portion to commence the exposure.
Upon the completion of exposure, the periodical transfer pulse CTG is outputted, so that the signal charges accumulated within the exposure time (shutter speed) IV are transferred to the vertical transfer portion to finish the exposure.
The unnecessary electrical charges transferred to the vertical transfer portion by the second compulsive transfer pulse PTG2 are swept within a short space of time in accordance with the sweep signal immediately before the periodical transfer pulse CTG is outputted.
The signal charges which have been accumulated in the light receiving portion within the exposure time TV and which are transferred to the vertical transfer portion in accordance with the periodical transfer pulse CTG are read by the periodical read signal to be recorded as picture signals (see C in FIG. 21).
Upon recording in the movie mode, as shown in FIG. 35, the unnecessary electrical charges which are accumulated in the light receiving portion in field n immediately before the exposure field n+1 are transferred to the vertical transfer portion by the periodical transfer pulse CTG1 at the commencement of the exposure in the exposure field n+1, so that the exposure can be commenced.
The unnecessary electrical charges which are transferred to the vertical transfer portion by the periodical transfer pulse CTG1 are read by the read signal before the periodical transfer pulse CTG2 at the completion of exposure is outputted and are discharged.
When the periodical transfer pulse CTG2 is outputted at the completion of exposure, the signal charges which are accumulated in the exposure field n+1, i.e., within the exposure time TV (.apprxeq.1/60 second), are transferred to the vertical transfer portion to finish the exposure. The signal charges transferred to the vertical transfer portion are read by the periodical read signal to be recorded as a picture signal (seed in FIG. 30).
As can be seen from the foregoing, in the movie mode, only the periodical read signals outputted between the periodical transfer pulse CTG1 at the commencement of the exposure and the periodical accumulation signal CTG0 outputted immediately before the periodical transfer pulse CTG1.
On the other hand, in the still mode, the first compulsive transfer pulse PTG1 and the sweep signal are outputted in field n immediately before field n+1 which includes the second compulsive transfer pulse PTG2 at the commencement of the exposure after the periodical read signal is outputted.
It can be understood from the above description that in the conventional CCD image pick-up unit, there is a difference in processing of the unnecessary electrical charges which are accumulated in the field immediately before the exposure between the movie mode and the still mode. Accordingly, the image quality of the picture recorded in the movie mode is different from the picture recorded in the still mode, even in the same exposure time.
This is the reason that there is a difference in image quality of the recorded pictures even with the same exposure value in a camera in which the picture can be taken at both the still mode and the movie mode.
Furthermore, in the case of a pure electronic shutter as mentioned above, the smear level in the movie mode in which the signal charges accumulated in the light receiving portion are periodically read is different from that in the still mode in-which the electrical charges are accumulated only for the time corresponding to the shutter speed at the operation of the electronic shutter and then the signal charges accumulated in that time are read, resulting in a difference in image quality.
For instance, in the movie mode, the periodical transfer pulses are outputted every 1/60 second to transfer the signal charges from the light receiving portion to the vertical transfer portion, and the signal charges are then read by the read signal. Therefore, the signal charges which can not be read in the previous frame may remain in the vertical transfer portion.
However, when a picture is taken at a 1/60 second shutter speed in the still mode, the electrical charges transferred to the vertical transfer portion are swept immediately before the periodical transfer pulse is outputted after the compulsive transfer pulse is outputted. Accordingly, almost no unnecessary electrical charges remain in the vertical transfer portion.
Namely, there is a tendency that the smear level in the movie mode is higher than that in the still mode, within the same electrical charge accumulation time. Such a smear level difference is undesirable and accordingly should be eliminated. To this end, the circuit adjustment is usually effected, which is however, troublesome and difficult.
The assignee of the present application has developed a driving apparatus of a image pick-up device for operating the electronic shutter, using the CCD image pick-up unit for a conventional movie. An electronic still camera using the driving apparatus of the image pick-up device will be explained below with reference to FIG. 18.
In the CCD image pick-up unit for a video camera, normally, the signal charges accumulated in the light receiving portion are periodically (approximately every 1/60 second) transferred to the vertical transfer portion at one time. The signal charges are successively read before the subsequent signal charges are periodically transferred. The unnecessary electrical charges when no record is effected are swept out by the sweep pulses. The transfer of the signal charges to the vertical transfer portion is effected by the transfer pulse, and the reading of the signal charges is effected by the read pulse including the vertical and horizontal transfer pulses. Furthermore, the sweeping is effected by the sweep pulse. When the electronic shutter operation is effected, the signal charges are transferred by the compulsive transfer pulse which are optionally outputted.
The transfer pulse, the read pulse and the sweep pulse are made of a combination of a control signal and a drive pulse. The control signal consists of one pulse, and the transfer pulse consists of at least four pulses. The control pulse and the transfer pulse for transferring the electrical charges of the light receiving portion to the vertical transfer portion, the read pulse and the sweep pulse are made of a combination of one accumulation control pulse and four transfer pulses. Namely, four transfer pulses selectively become a transfer pulse, read pulse or sweep pulse in accordance with the level of the control pulse.
In the driving apparatus of the image pick-up device mentioned above, the control pulse and the transfer pulse for the operation of the electronic shutter are processed by a control device (i.e., microcomputer). Therefore, seven pulses; including one accumulation control pulse, at least four transfer pulses, one sweep demand pulse and one switching pulse are outputted from the microcomputer to operate the electronic shutter. Namely, seven output ports are used in the microcomputer.
However, the usage of the seven output ports of the microcomputer (MPU, CPU etc.) decreases the number of output ports which can be used for other control purposes. In addition, a software for producing the transfer pulses is also necessary.
Furthermore, if a conventional CCD image pick-up unit is used to realize an electronic shutter, a smear would appear on a picture. For instance, in the CCD image pick-up device 11 shown in FIGS. 2A and 2B, if relatively high intensity of light is incident upon a certain photoelectric element (photodiode) 11a and a vertical transfer portion (vertical transfer CCD) 11b adjacent thereto, in comparison with light incident upon the other photoelectric elements 11a and vertical transfer portion's 11b, that is, if an object of high luminance is imaged on a certain photoelectric element 11a and a vertical transfer portions 11b adjacent thereto, unnecessary electric charges which overflow from the adjacent photodiodes 11a or which are caused by light passing through the edge portions of the shield member 73 or transmitted through the shield member 73 are accumulated on that vertical transfer portions 11b. The signal charges which are transferred to the vertical transfer portions 11b are mixed with the unnecessary (harmful) electric charges, so that a bright vertical line appears on an image below the photodiode 11a (FIGS. 33A and 34A). This is a smear.
To prevent the occurrence of such a smear, it is known to control the exposure time TV by the opening and closing operation of a shutter. However, the provision of the shutter increases the number of components, thus resulting in a heavy and large camera. In addition, the shutter makes the control circuit of the camera more complicated.
The assignee of the present application has proposed Japanese Patent Application Serial No. 63-317904 which discloses means for preventing the occurrence of smear, in which the signal charges are read after the diaphragm is closed. In particular, in case of continuous photographing, the diaphragm and the shutter must be closed at very high speed. However, the repeated quick closing of the diaphragm results in an increased electrical power consumption.